Rearranged oleananes in Tertiary oils from the Baiyun Sag, Pearl River Mouth Basin, South China Sea: identification, distribution and geochemical significance

Abstract

Three abundant rearranged oleananes detected in Tertiary oils from the Baiyun Sag, South China Sea, were designated as 5(4 → 3) abeo-3α(H), 5β(H), 18α(H)-oleanane (I), 3α, 5β-dimethyl-23α, 25-dinor-10β(H), 18α(H)-oleanane (II) and 1(10 → 5)abeo-3β-methyl-24β-nor-18α(H)-oleanane (III), using gas chromatography-mass spectrometry (GC–MS) and GC–MS–MS. The relative abundances of different rearranged oleananes correlated with each other, and also showed strong positive correlations with those of oleanane or des-A-oleanane. This is consistent with rearranged oleananes, oleanane (OL) and des-A-oleanane originating from functionalized oleanane triterpenoids, but by different formation processes. The distribution of rearranged oleananes in oils from the study area is controlled mainly by angiosperm inputs and the diagenetic conditions of their source rocks, rather than thermal maturity. Oligocene marine transgression in the study area may have played an important role in the expression of saturated oleanoids. Moreover, oxic and acidic peat swamp conditions in the Panyu Low Uplift favoured the formation of rearranged oleananes compared to oleananes. In addition, oils from the Baiyun Sag can be classified into two oil families, i.e., A and B, using the relative abundance of rearranged oleananes and other source-related indicators. The Family A oils in the eastern and northeastern portions of the sag are characterized by relatively low pristane/phytane (Pr/Ph) ratios, low rearranged oleananes/oleanane ((I + II + III)/OL) and bicadinane-T/C30 hopane (T/C30H) ratios. The Family B oils in the Panyu Low Uplift and northeastern portion of the sag have higher Pr/Ph, (I + II + III)/OL and T/C30H ratios. The related parameters of rearranged oleananes were therefore shown to be effective indicators for oil-oil or oil-source rock correlation in the study area.